Coatings to control infrared reflection and absorption are highly desirable, especially where the emissivity and reflectivity of surfaces are significant features when it relates to detection by optical (infrared (“IR”) and visible light) seekers. Coatings are often designed to provide particular emissivity/reflectivity characteristics. However, it is difficult to accurately measure the optical characteristics of these coating material (and hence to design a material for a particular purpose).
Genetic algorithms for design optimizations have made possible a new generation of optical coatings to control infrared reflection and absorption. Configurations suitable for this purpose generally are applied to thick substrates, but otherwise are just IR versions of the Fabry-Perot filter. These configurations incorporate layers, with thicknesses on the order of 1 μm to exploit interference effects. Although an extensive IR-optical data base is now available for design purposes such data for organic materials remains relatively scarce. Tests on thin polymeric films can supply optical properties for design of coatings containing both organic and inorganic layers and also provide useful operational data on the spectro-photometric devices employed.
The Hemispherical Directional Reflectometer (HDR) is a convenient instrument for optical characterization. HDR measurements provide broadband IR data for oblique polarized reflection, as well as normal-incidence transmission. Tests on thin polymer films in low-loss wavelength ranges typically show fringes conforming to Fresnel reflection/transmission. Hence, HDR measurements are a promising approach to determine the optical constants of organic materials. The same experiments also quantify operational features of the HDR for application to inorganic materials.
However, the HDR has some limitations in determining the optical constants of materials. The achievable angular resolution is limited by the HDR configuration. In particular, an overhead mirror used to collect IR radiation scattered from a sample film subtends a non-negligible angle. This effect causes measured reflection extrema to be “damped” relative to rigorous calculations assuming incidence at a discrete angle. Currently existing data-reduction algorithms for deriving the optical constants “n” (refractive index) and “k” (absorptive index) of a material, such as a thin film, do not correct for this observed angular spread. As a result, HDR measurements of the optical constants for a material, as well as for the thickness of the material, lack the accuracy required to develop today's high-tolerance coatings.
Therefore, a need exists for a method and system for correcting angular spread in HDR determination of IR optical constants that can reduce or eliminate the accuracy problems of prior art HDR methods and systems.